Fusion creates all elements heavier than hydrogen and powers stars across the universe. This process combines the atomic nuclei of lighter elements into a single, larger nucleus, releasing tremendous energy. The necessary conditions—immense temperature and pressure—are naturally met within the dense cores of stars. Hydrogen, the most abundant element, is the initial fuel. Elements from helium to uranium are forged through a sequence of fusion events and nuclear processes as stars live and die.
The Stellar Cradle: Fusion of Light Elements
The process begins with hydrogen fusing into helium, defining the majority of a star’s lifetime. In stars similar to the Sun, this occurs via the Proton-Proton Chain (P-P Chain). This reaction starts when two protons (hydrogen nuclei) fuse, forming a deuterium nucleus, a positron, and a neutrino.
Deuterium quickly captures another proton to form helium-3. Finally, two helium-3 nuclei combine to create stable helium-4, releasing two protons that restart the cycle. This slow process allows stars like the Sun to burn steadily for billions of years.
In more massive and hotter stars, the Carbon-Nitrogen-Oxygen (CNO) cycle dominates the conversion of hydrogen to helium. This cycle uses carbon, nitrogen, and oxygen as catalysts, which are consumed and then regenerated. The CNO cycle is highly temperature-sensitive, requiring core temperatures above 20 million Kelvin to be efficient.
Building the Intermediate Elements: Carbon and Oxygen
When a star exhausts its core hydrogen, gravitational forces cause the core to contract and heat up dramatically. If the core temperature reaches about 100 million Kelvin, the star begins fusing the accumulated helium ash. This phase, known as helium burning, typically occurs when the star has evolved into a red giant.
The primary reaction during helium burning is the Triple-Alpha Process, which creates carbon. Two helium nuclei (alpha particles) first fuse to form unstable beryllium-8, which decays almost instantly. Under the core’s high density and temperature, a third alpha particle strikes the beryllium-8 before it decays, producing stable carbon-12. This reaction is possible due to a specific energy resonance in the carbon-12 nucleus, predicted by astrophysicist Fred Hoyle.
After carbon forms, subsequent reactions build heavier elements by adding more helium nuclei. For example, carbon-12 fuses with helium to create oxygen-16. Oxygen-16 can then combine with helium to form neon-20, continuing the element sequence.
The Iron Barrier: Fusion in Massive Stars
Only stars significantly more massive than the Sun (eight solar masses or greater) can achieve the extreme conditions needed to fuse elements heavier than carbon and oxygen. After helium depletion, the core undergoes a rapid series of sequential fusion stages. Each stage requires a progressively hotter and denser core, leading to the fusion of carbon into neon, neon into oxygen, and oxygen into silicon.
This layering of reactions gives the star an “onion-skin” structure, concentrating the heaviest elements in the center. The final stage is Silicon Burning, occurring at temperatures of several billion Kelvin. This process rapidly transforms silicon and sulfur into elements near the peak of nuclear stability, primarily iron-56 and nickel-56.
Iron-56 represents a fundamental limit because its nucleus possesses the maximum binding energy per nucleon. Fusing elements lighter than iron releases energy, supporting the star against gravity. However, fusing iron into a heavier element consumes energy instead of releasing it. This lack of energy generation causes the gravitational pressure to become unbalanced, leading to the catastrophic collapse of the iron core within a fraction of a second.
Beyond Iron: Neutron Capture and the Cosmic Forges
Elements heavier than iron cannot be created through standard exothermic fusion. They are formed primarily through neutron capture processes in explosive environments, which bypass the iron energy barrier by bombarding existing nuclei with free neutrons. There are two main pathways: the slow process (s-process) and the rapid process (r-process).
The Slow Process (s-process)
The s-process involves capturing neutrons slowly enough to allow unstable isotopes to undergo beta decay before capturing another neutron. This process occurs in the outer layers of moderate-mass stars, specifically in asymptotic giant branch (AGB) stars. The s-process creates about half of the elements heavier than iron, including lead and bismuth, resulting in isotopes closer to the valley of stability.
The Rapid Process (r-process)
The r-process is characterized by a rapid, intense flood of neutrons, causing nuclei to capture many neutrons faster than they can decay. This mechanism creates the heaviest, most neutron-rich elements, such as gold, platinum, thorium, and uranium. The necessary high neutron density is provided by catastrophic cosmic events, including supernova explosions or the merger of two neutron stars. These violent events disperse the newly forged heavy elements into the cosmos, completing the cycle of element creation.